The demand for the ability for computing label switched paths (LPSs) that span across multiple areas or multiple autonomous systems has evolved from an academic discussion to a feature request of a carrier network. There have been a few solutions that have been proposed for computing constraint based LSPs that span across multiple areas or multiple autonomous systems.
In one proposed solution, a global TE (traffic engineering) database is used. While the global TE database is a simple solution for computing LSPs that span across multiple areas or multiple ASes, it is prohibitive because the global TE database may be too large and therefore negates the purpose of having multiple areas or multiple ASes, and it violates the information hiding and confidentiality requirement, which is unacceptable by Internet Service Providers (ISPs).
Another proposed solution uses a crankback method. The crankback method is more practical than the global TE database solution as it is an exhaustive search based mechanism and will find an LSP if it exists. However, the crankback method also includes obvious drawbacks. One drawback is that the crankback method does not scale because it often requires, and therefore wastes, more than one tryout to find a qualified LSP; and it is RSVP (Resource Reservation Protocol) signaling based which is by its nature poor in scaling. Another drawback is that the extra signaling messages used in the crankback method adds burdens on the existing network. Another drawback is that the path, if found, is not guaranteed to be optimal. Another drawback is that the crankback method is labor intensive in that it requires many manual configurations to specify border routers. Finally, another drawback is that the crankback method requires substantial RSVP changes, both in protocol and operation.
Another proposed solution is described in RFC (Request For Comments) 5441, “A Backward-Recursive PCE-Based Computation (BRPC) Procedure to Compute Shortest Constrained Inter-Domain Traffic Engineering Label Switched Paths,” April 2009. The solution proposed in RFC 5441 assumes that the destination is known in a particular domain and area; however this assumption is not always true. Moreover, the destination may be multi-homed (meaning reachable through different areas and domains), which the RFC 5441 method cannot handle. Furthermore, the method described in RFC 5441 mandates a PCEP (Path Computation Element Protocol) extension which understands the Virtual Shortest Path Tree (VSPT), which further complicates the method. Moreover, the VSPT approach only addresses one destination at a time.